Swept frequency measurement systems are used to characterize the frequency response of a variety of electronic devices such as amplifiers, filters, attenuators and systems using such components. A swept frequency measurement system usually includes a sweeping signal generator which supplies a signal that changes frequency linearly with time. The signal is supplied to a device under test and the output of the device under test is displayed as either absolute or relative magnitude or phase as a function of frequency. Since it is desirable to have an accurately calibrated frequency axis in such a display, a number of methods have been used in the prior art.
The display itself may be marked with graduations, such as a graticule on a cathode ray tube (CRT), that are calibrated according to frequency. This method has the disadvantage that a change in the calibration or the frequency range of the sweeping signal generator will make the display uncalibrated and cannot be detected without the aid of additional test equipment. This limitation is significant because the range over which the sweeping signal generator sweeps may be easily changed on most commercially available generators.
A second method of providing frequency calibration on a swept frequency measurement system display is to use one or more CW signals which are shown simultaneously or alternatively on the display. These signals may also be used to produce markers on the display, thus serving much the same function as a calibrated graticule on a CRT. This method has the disadvantage that it is still not possible to know the frequency of a point between the markers accurately, since interpolation is required.
A third method of providing frequency information in a swept frequency measurement system involves the use of a counter which is used to count the frequency of the sweeping signal. In order to count the signal frequency at a particular point, the sweeping of the signal generator is stopped before the counter gate is opened. The counter then counts the frequency of the signal at the marker, and after the count has been terminated the sweep is started again. In this method it is necessary to stop the sweep since simply counting the output signal from the sweeping signal generator would not produce an accurate result because the signal is continuously changing frequency. Such a system is described, for example, in U.S. Pat. No. 3,643,126 issued to Robert R. Hay, Feb. 15, 1972. This system has the disadvantage, however, that it is necessary to stop the sweep of the sweeping signal generator while the count is made, thus disrupting the measurement and requiring extra control lines between the counter and the sweeping signal generator.